Showmax has a number of features to help you limit the amount of data you're using for streaming.
Managing your data settings
You can limit the amount of data used per hour of streaming or restrict streaming to Wi-Fi only within your Account. For more information, take a look at How do I manage my streaming data settings?
Using downloads
The Showmax app allows you to download content while you're on Wi-Fi, to watch later when you're not. For more information, take a look at How to download content on your device.